RFR: 8242427: JVMTI frame pop operations should use Thread-Local Handshakes
Yasumasa Suenaga
suenaga at oss.nttdata.com
Fri Aug 28 01:24:04 UTC 2020
Hi David,
On 2020/08/27 15:49, David Holmes wrote:
> Sorry I just realized I reviewed version 00 :(
>
> I have concerns with the added locking:
>
> MutexLocker mu(JvmtiThreadState_lock);
>
> Who else may be holding that lock? Could it be our target thread that we have already initiated a handshake with? (The lock ranking checks related to safepoints don't help us detect deadlocks between a target thread and its handshaker. :( )
I checked source code again, then I couldn't find the point that target thread already locked JvmtiThreadState_lock at direct handshake.
> It is far from clear now which functions are reachable from handshakes, which from safepoint VM_ops and which from both.
>
> ! assert(SafepointSynchronize::is_at_safepoint() || JvmtiThreadState_lock->is_locked(), "Safepoint or must be locked");
>
> This can be written as:
>
> assert_locked_or_safepoint(JvmtiThreadState_lock);
>
> or possibly the weak variant of that. ('m puzzled by the extra check in the strong version ... I think it is intended for the case of the VMThread executing a non-safepoint VMop.)
> JvmtiEventController::set_frame_pop(), JvmtiEventController::clear_frame_pop() and JvmtiEventController::clear_to_frame_pop() are no longer called at safepoint, so I remove safepoint check from assert() in new webrev.
webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/webrev.03/
diff from previous webrev: http://hg.openjdk.java.net/jdk/submit/rev/2a2c02ada080
Thanks,
Yasumasa
> Thanks,
> David
> -----
>
>
> On 27/08/2020 4:34 pm, David Holmes wrote:
>> Hi Yasumasa,
>>
>> On 27/08/2020 9:40 am, Yasumasa Suenaga wrote:
>>> Hi David,
>>>
>>> On 2020/08/27 8:09, David Holmes wrote:
>>>> Hi Yasumasa,
>>>>
>>>> On 26/08/2020 5:34 pm, Yasumasa Suenaga wrote:
>>>>> Hi Patricio, David,
>>>>>
>>>>> Thanks for your comment!
>>>>>
>>>>> I updated webrev which includes the fix which is commented by Patricio, and it passed submit repo. So I switch this mail thread to RFR.
>>>>>
>>>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8242427
>>>>> webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/webrev.00/
>>>>>
>>>>> I understand David said same concerns as Patricio about active handshaker. This webrev checks active handshaker is current thread or not.
>>>>
>>>> How can the current thread already be in a handshake with the target when you execute this code?
>>>
>>> EnterInterpOnlyModeClosure might be called in handshake with UpdateForPopTopFrameClosure or with SetFramePopClosure.
>>>
>>> EnterInterpOnlyModeClosure is introduced in JDK-8238585 as an alternative in VM_EnterInterpOnlyMode.
>>> VM_EnterInterpOnlyMode returned true in allow_nested_vm_operations(). Originally, it could have been called from other VM operations.
>>
>> I see. It is a pity that we have now lost that critical indicator that shows how this operation can be nested within another operation. The possibility of nesting is even more obscure with JvmtiEnvThreadState::reset_current_location. And the fact it is now up to the caller to handle that case explicitly raises some concern - what will happen if you call execute_direct whilst already in a handshake with the target thread?
>>
>> I can't help but feel that we need a more rigorous and automated way of dealing with nesting ... perhaps we don't even need to care and handshakes should always allow nested handshake requests? (Question more for Robbin and Patricio.)
>>
>> Further comments:
>>
>> src/hotspot/share/prims/jvmtiEnvThreadState.cpp
>>
>> 194 #ifdef ASSERT
>> 195 Thread *current = Thread::current();
>> 196 #endif
>> 197 assert(get_thread() == current || current == get_thread()->active_handshaker(),
>> 198 "frame pop data only accessible from same thread or direct handshake");
>>
>> Can you factor this out into a separate function so that it is not repeated so often. Seems to me that there should be a global function on Thread: assert_current_thread_or_handshaker() [yes unpleasant name but ...] that will allow us to stop repeating this code fragment across numerous files. A follow up RFE for that would be okay too (I see some guarantees that should probably just be asserts so they need a bit more checking).
>>
>> 331 Handshake::execute_direct(&op, _thread);
>>
>> You aren't checking the return value of execute_direct, but I can't tell where _thread was checked for still being alive ??
>>
>> ---
>>
>> src/hotspot/share/prims/jvmtiEventController.cpp
>>
>> 340 Handshake::execute_direct(&hs, target);
>>
>> I know this is existing code but I have the same query as above - no return value check and no clear check that the JavaThread is still alive?
>>
>> ---
>>
>> Do we know if the existing tests actually test the nested cases?
>>
>> Thanks,
>> David
>> -----
>>
>>>
>>> Thanks,
>>>
>>> Yasumasa
>>>
>>>
>>>> David
>>>> -----
>>>>
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Yasumasa
>>>>>
>>>>>
>>>>> On 2020/08/26 10:13, Patricio Chilano wrote:
>>>>>> Hi Yasumasa,
>>>>>>
>>>>>> On 8/23/20 11:40 PM, Yasumasa Suenaga wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> I want to hear your opinions about the change for JDK-8242427.
>>>>>>>
>>>>>>> I'm trying to migrate following operations to direct handshake.
>>>>>>>
>>>>>>> - VM_UpdateForPopTopFrame
>>>>>>> - VM_SetFramePop
>>>>>>> - VM_GetCurrentLocation
>>>>>>>
>>>>>>> Some operations (VM_GetCurrentLocation and EnterInterpOnlyModeClosure) might be called at safepoint, so I want to use JavaThread::active_handshaker() in production VM to detect the process is in direct handshake or not.
>>>>>>>
>>>>>>> However this function is available in debug VM only, so I want to hear the reason why it is for debug VM only, and there are no problem to use it in production VM. Of course another solutions are welcome.
>>>>>> I added the _active_handshaker field to the HandshakeState class when working on 8230594 to adjust some asserts, where instead of checking for the VMThread we needed to check for the active handshaker of the target JavaThread. Since there were no other users of it, there was no point in declaring it and having to write to it for the release bits. There are no issues with having it in production though so you could change that if necessary.
>>>>>>
>>>>>>> webrev is here. It passed jtreg tests (vmTestbase/nsk/{jdi,jdwp,jvmti} serviceability/{jdwp,jvmti})
>>>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8242427/proposal/
>>>>>> Some comments on the proposed change.
>>>>>>
>>>>>> src/hotspot/share/prims/jvmtiEnvThreadState.cpp, src/hotspot/share/prims/jvmtiEventController.cpp
>>>>>> Why is the check to decide whether to call the handshake or execute the operation with the current thread different for GetCurrentLocationClosure vs EnterInterpOnlyModeClosure?
>>>>>>
>>>>>> (GetCurrentLocationClosure)
>>>>>> if ((Thread::current() == _thread) || (_thread->active_handshaker() != NULL)) {
>>>>>> op.do_thread(_thread);
>>>>>> } else {
>>>>>> Handshake::execute_direct(&op, _thread);
>>>>>> }
>>>>>>
>>>>>> vs
>>>>>>
>>>>>> (EnterInterpOnlyModeClosure)
>>>>>> if (target->active_handshaker() != NULL) {
>>>>>> hs.do_thread(target);
>>>>>> } else {
>>>>>> Handshake::execute_direct(&hs, target);
>>>>>> }
>>>>>>
>>>>>> If you change VM_SetFramePop to use handshakes then it seems you could reach JvmtiEventControllerPrivate::enter_interp_only_mode() with the current thread being the target.
>>>>>> Also I think you want the second expression of that check to be (target->active_handshaker() == Thread::current()). So either you are the target or the current active_handshaker for that target. Otherwise active_handshaker() could be not NULL because there is another JavaThread handshaking the same target. Unless you are certain that it can never happen, so if active_handshaker() is not NULL it is always the current thread, but even in that case this way is safer.
>>>>>>
>>>>>> src/hotspot/share/prims/jvmtiThreadState.cpp
>>>>>> The guarantee() statement exists in release builds too so the "#ifdef ASSERT" directive should be removed, otherwise "current" will not be declared.
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>> Patricio
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Yasumasa
>>>>>>
More information about the serviceability-dev
mailing list